Hi Mari,

The tax calculation currently depends on Line Item and Shipping Address - for both sale tax or shipping tax. When line item is tax category A, we have both sale tax and shiping tax for category A. And line item is category B, we should define both sale tax and shipping tax for category B. This is default (for US market).

So in this case, defining a shipping tax for category B is one of solutions.

However, the tax calculation was flexible, we won't know what way is correct or not. It depends on market, US is different from EU or Asia - that's the reason we allow customizing tax calculation. You could customze tax calculation in your own way, in market you support, in your bussiness.

We could design tax calculation doesn't depend on item tax category, it depends on item type instead. For example, Fashion type has tax = 10%, Shoes has tax = 5% and Hat has tax = 7%...

Or we don't need customers create the order Shipping Adderss for tax calculating, we could use their prefer address to calculate tax, ...

That means it could be your choice, your way.

For customizing, you could prefer to this document http://world.episerver.com/documentation/Class-library/?documentId=commerce/9/7426E63C 

Hope this helps!

/Son Do

Fyi: I changed the implementation to fetch shipping tax based on tax type (independent of shipment address and line items).